Efforts continued to develop a high-sensitivity projection imaging system for visualizing in vivo positron-emitting radiopharmaceuticals in human and animal tumors. A small group of female subjects with breast cancer were imaged with the prototype system, reported in the previous period, after administration of fluorodeoxyglucose labeled with F- 18. In most of these subjects, images of the breast lesions were identified in accord with positron emission tomography (PET) scans obtained from the same subjects. However, in some patients, tumors were not identified because of the tumor's location (against the chest wall) or because the tumor fell outside the small field of view of the imaging device. Projection imaging studies in nude mice implanted with human breast tumors continued throughout the reporting period, and a number of new, tumor-seeking PET radiopharmaceuticals were successfully evaluated. Modifications to this imaging system to improve performance were tested by computer simulation, and a new system based on these findings is now being assembled. The new system should exhibit better spatial resolution and a greater count rate capability than the present device, thereby improving the quantitative accuracy of both human and animal studies.